Which of the following is known as suicidal enzyme? (AIIMS November 2013, May 2013)

Okay, let's tackle this question about the "suicidal enzyme." First, I need to remember what a suicidal enzyme is. From what I recall, suicidal enzymes are those that get inactivated during the process of catalyzing a reaction. They form a covalent bond with the substrate, which leads to their own inactivation. This is different from typical enzymes that remain unchanged after the reaction.

Now, the options aren't provided here, but common enzymes associated with this term might include acetylcholinesterase, which breaks down acetylcholine and gets inhibited in the process when exposed to certain poisons. Another possibility could be alcohol dehydrogenase, but I'm not sure if that's considered a suicidal enzyme. Maybe some other enzymes involved in detoxification processes?

Wait, I think the classic example of a suicidal enzyme is acetylcholinesterase. When it hydrolyzes acetylcholine, it normally works, but in the presence of organophosphates, it forms a phosphorylated enzyme that's inactive. This is a key point in organophosphate poisoning. So the correct answer would be acetylcholinesterase. Let me confirm that.

The question is from AIIMS, so it's important to be precise. The term "suicidal enzyme" is definitely linked to acetylcholinesterase in the context of irreversible inhibition by organophosphates. That makes sense because the enzyme's active site is covalently modified, leading to permanent inactivation. Other enzymes don't get inactivated in this way during normal function unless there's a specific inhibitor involved.

So the correct answer should be acetylcholinesterase. The other options might include enzymes like alcohol dehydrogenase, but they don't fit the suicidal enzyme definition. Alternatively, maybe another enzyme like butyrylcholinesterase, but acetylcholinesterase is more specific. I need to make sure that there's no other enzyme that's commonly referred to by that term. I think that's the main one.

**Core Concept**
The term "suicidal enzyme" refers to an enzyme that becomes irreversibly inactivated during catalysis, often due to covalent modification by the substrate or a toxic metabolite. This mechanism is critical in toxicology and enzyme inhibition pathways.

**Why the Correct Answer is Right**
Acetylcholinesterase (AChE) is the classic example of a suicidal enzyme. It hydrolyzes acetylcholine at neuromuscular junctions but is irreversibly inhibited by organophosphates (e.g., nerve agents). These toxins form a stable phosphorylated complex with AChE, permanently deactivating it—a process termed "suicidal inactivation." This mechanism underlies organophosphate poisoning and resistance to cholinergic neurotransmission.

**Why Each Wrong Option is Incorrect**
**Option A:** Alcohol dehydrogenase is not a suicidal enzyme; it metabolizes ethanol without self-inactivation.
**Option B:** Cytochrome P450 enzymes typically catalyze oxidation reactions and remain functional post-reaction.
**Option C:** Lactate dehydrogenase (LDH) is a metabolic enzyme involved in glycolysis and does not exhibit suicidal inactivation.
**Option D:** Carbonic anhydrase catalyzes CO₂ hydration and retains activity after each cycle

✓ Correct Answer: B. Cyclooxygenase